258 The North-West Lunar Limb. 



the moon presented no other type, the idea of an eruptive force 

 would scarcely have been entertained. 



A very natural, though perfectly hopeless feeling of curi- 

 osity is sometimes expressed, as to what can be at the back 

 of the moon, in that portion of her globe which is for ever 

 hidden from mortal gaze ; and it becomes in proportion inte- 

 resting to ascertain what may exist upon the limb, and to 

 watch the transition from the known to the unknown. If this 

 does not always intimate the probability of perfect similarity 

 (and there may be reason for believing that it does not always), 

 in some instances it does so very distinctly; and such an 

 instance comes before us on this NW. limb which we have been 

 hitherto following, in the great plain lying immediately oppo- 

 site to Endymion, and called 



Mare Humboldtianum. Of this B. and M. claim the dis- 

 covery ; but it appears distinctly enough in the map of their 

 contemporary,* Lohrmann ; and there is some indication of it, 

 though placed too far N., on the lunar globe of Eussell. It is 

 a grey level, exactly corresponding with the other " seas" in 

 character, and extending upwards of 190 miles in length : the 

 breadth is, of course, very difficult to ascertain, but it appears 

 to be still greater, so that its area must be something more than, 

 half that of the Mare Grisium, and not far from that of the 

 Mare Vajporum. In the most unfavourable circumstances of 

 libration it is invisible, its nearer boundary falling upon the 

 limb ; in an opposite state the whole comes into view, and the 

 lofty mountains at its back form the profile of the limb. These 

 are of very considerable height — probably in one part not less 

 than 16,000 feet; and in one place there is a gap, showing, 

 perhaps, a communication opening to some more distant level. 

 Its colour is evidently grey, though for reasons of perspective 

 this is less decided in proportion to its nearness to the limb. 

 B. and M. have given a separate plate of its most favourable 

 projection in " Der Mond." Long low ridges alone are per- 

 ceptible in its interior. 



1863, Sept. 20, the Eev. H. C. Key, in examining the 

 moon with a 12-inch glass speculum of his own workmanship, 

 on which the silver film had not as yet been deposited, detected 

 several extensive flattenings of the limb. Of some of these 

 we shall have to speak more particularly at a future time ; but 

 it must be observed in this place, that in consequence of his 

 information I looked for these curious objects on Sept. 23, and 

 saw, besides two on the WSW. limb, two others in the region 



* The dates of these modern Selenographies are" as follows": — Lohrmann's 

 map, from observations 1822-1836. His " Topographie," Part I. (all that ever 

 appeared), 1824. First Quadrant of Beor and Miidler's Map, 1834. Their 

 " Mond" (Moon), 1837. 



